Abiotic stress is the primary cause of crop loss worldwide, causing average yield losses of more than 50% for major crops (Boyer, J. S. (1982) Science 218:443-448; Bray, E. A. et al. (2000) In Biochemistry and Molecular Biology of Plants, Edited by Buchannan, B. B. et al., Amer. Soc. Plant Biol., pp. 1158-1249). Among the various abiotic stresses, drought is the major factor that limits crop productivity worldwide. Exposure of plants to a water-limiting environment during various developmental stages appears to activate various physiological and developmental changes. Understanding of the basic biochemical and molecular mechanism for drought stress perception, transduction and tolerance is a major challenge in biology. Reviews on the molecular mechanisms of abiotic stress responses and the genetic regulatory networks of drought stress tolerance have been published (Valliyodan, B., and Nguyen, H. T., (2006) Curr. Opin. Plant Biol. 9:189-195; Wang, W., et al. (2003) Planta 218:1-14); Vinocur, B., and Altman, A. (2005) Curr. Opin. Biotechnol. 16:123-132; Chaves, M. M., and Oliveira, M. M. (2004) J. Exp. Bot. 55:2365-2384; Shinozaki, K., et al. (2003) Curr. Opin. Plant Biol. 6:410-417; Yamaguchi-Shinozaki, K., and Shinozaki, K. (2005) Trends Plant Sci. 10:88-94).
It is well known that responses to abiotic stress vary significantly among plant species and among varieties and cultivars within a plant species. Certain species, varieties or cultivars are more tolerant to abiotic stress such as drought than others. The genotypes of such plants are attractive sources of genes involved in unique responses to abiotic stress. Identification of stress response genes and expression of them in transgenic plants have been tried quite extensively to date. However, stress response genes introduced into plants are often not expressed very well. Reasons for the poor expression may include inappropriate choice of promoters and/or other regulatory elements and destruction of exon-intron structure. Introduction of a plant genomic segment, which retains the native promoter, entire coding region and intact exon-intron structure, into plants may be an effective approach for good expression of a foreign stress responsive gene. For example, it was reported that an enzyme involved in photosynthesis was expressed much higher from a genomic clone than from a corresponding cDNA clone in rice (Ku et al. Nature Biotechnol. 17:76-80, 1999).
Recently, a method for efficient screening of genomic DNA fragments capable of providing plants with an agriculturally advantageous phenotypic variation was developed (U.S. Patent Publication No. US2008/0301832A1). In this method, plants are transformed with genomic fragments from a genomic library constructed from a higher plant, and the resultant transgenic plants are screened for an agriculturally advantageous phenotypic variation. The resultant plants could be screened for a unique response to abiotic stress, such as drought tolerance, and eventually, a genomic fragment, which may carry a stress responsive gene readily expressible in plants, may be identified. In order to identify a unique stress responsive gene and utilize this gene in transgenic plants, considerable experimentation is required. Among the many factors to consider include the following: choice of a plant from which a genomic library is constructed; how the transgenic plants are screened; how the genomic fragments are examined; and how the a stress responsive gene is pinpointed, characterized and used.